This invention relates to fuel supply control systems for automotive engines, and more particularly to fuel supply control systems which are capable of judging the property of the fuel (e.g., the 50 percent distillation temperature which represents the volatility of the fuel, or the octane number which measures the anti-knock property of the fuel) and adjusting the amount of fuel supplied to the engine on the basis of the judgment.
A control device for an automotive engine which judges the property of the fuel is disclosed by Japanese Laid-Open Patent, (Kokai) No. 2-201045. The device includes an exhaust gas sensor and a linear air/fuel ratio sensor. The exhaust gas sensor detects the concentration of a particular gas (the oxygen gas O.sub.2) and outputs a signal whose level is inverted each time when the ratio of the air/fuel mixture supplied to the engine crosses the theoretical air/fuel ratio (i.e., when the air/fuel ratio falls from the rich to the lean, or rises from the lean to the rich, across the theoretical). The linear air/fuel ratio sensor outputs a signal whose level is proportional to the air/fuel ratio, which is measured by the oxygen gas concentration of the exhaust gas. Each time the output of the exhaust gas sensor is inverted, the control means for controlling the air/fuel ratio judges the property of the fuel on the basis of the output of the linear air/fuel ratio sensor.
In the case of the device according to Japanese Laid-Open Patent (Kokai) No. 2-201045, the control means receives outputs from the exhaust gas sensor and the linear air/fuel ratio sensor, and reads the state of the output signal of the linear air/fuel ratio sensor when the output of the exhaust gas sensor is inverted. FIG. 6 is a diagram showing the levels of the outputs of the exhaust gas sensor and the linear air/fuel ratio sensor disclosed in Japanese Laid-Open Patent (Kokai) No. 2-201045. The theoretical air/fuel ratios .lambda..sub.R and .lambda..sub.H for the regular fuel and the high-octane-number fuel, respectively, are different. When the fuel is of the regular type, the output of the linear air/fuel ratio sensor is inverted at the theoretical air/fuel ratio .lambda..sub.R (solid curve). When, on the other hand, the fuel is of a high-octane-number type, the output of the linear air/fuel ratio sensor is inverted at the theoretical air/fuel ratio .lambda..sub.H (dotted curve). Thus, the control means judges the property of the fuel on the basis of the output level, V.sub.H and V.sub.R, of the linear air/fuel ratio sensor at the inversion of the output of the exhaust gas sensor, which varies depending on the property of the fuel.
Another control device for an automotive engine is disclosed in Japanese Laid-Open Patent (Kokai) No. 4-294266. Using the proportional-plus-integral control method, the control device controls the amount of the fuel injected into the engine to adjust the air/fuel ratio to the target level. The control is based on the difference between the target and the actual air/fuel ratio fed back from the air/fuel ratio sensor. The control periods, f40 and f80, of the proportional-plus-integral control at the low and the high water temperatures, 40.degree. C. and 80.degree. C., of the engine are measured, and the difference .DELTA.f80-40 between the control periods f80 and f40 at the two temperatures is calculated. Assuming that the operating conditions of the engine other than the coolant water temperature are the same, the difference .DELTA.f80-40 is smaller when the fuel is of the lighter quality (i.e., more volatile) than when the fuel is of the heavier quality (i.e., less volatile). Thus, the property or the volatility of the fuel can be determined from the difference between the control periods of the proportional-plus-integral control at the low and the high water temperatures.
The control device disclosed in the first-mentioned Japanese Laid-Open Patent (Kokai) No. 2-201045 has the following disadvantage. The theoretical air/fuel ratio depends on the property of the fuel. However, when the air/fuel mixture is combusted at the theoretical air/fuel ratio, the oxygen gas concentration contained in the exhaust gas is at the same level irrespective of the property of the fuel. Thus, if the output of the linear air/fuel ratio sensor detecting the oxygen concentration contained in the exhaust gas is read in by the control means upon inversion of the air/fuel ratio across the theoretical ratio, the output of the linear air/fuel ratio sensor read in by the control means remains at the same level irrespective of the property of the fuel. Thus, the output level of the linear air/fuel ratio sensor does not vary as shown in FIG. 6. Rather, irrespective of the variation of the theoretical air/fuel ratio due to the difference in the property of the fuel, the output of the linear air/fuel ratio sensor is at the same level at the inversion of the air/fuel ratio across the theoretical, provided that the linear air/fuel ratio sensor detects the oxygen concentration contained in the exhaust gas. Thus, an accurate judgment of the property of the fuel is difficult.
The control device of the second-mentioned Japanese Laid-Open Patent (Kokai) No. 4-294266, on the other hand, has the following disadvantage. The property of the fuel is determined from the difference between the control periods of the proportional-plus-integral control at the low and the high water temperatures of the engine, 40.degree. C. and 80.degree. C. The control period depends, however, not only on the water temperature but also on other operating condition of the engine. Thus the difference between the control periods at the low and the high water temperatures does not represent the property of the fuel accurately.